The following method is for setting the transfer type of an FTP connection. Basically, I'd like to validate the character input (see comments).

Is this going overboard? Is there a more elegant approach? How do you approach parameter validation in general? Any comments are welcome.

public void setTransferType(Character typeCharacter,
        Character optionalSecondCharacter) throws NumberFormatException,
        IOException {

    // http://www.nsftools.com/tips/RawFTP.htm#TYPE
    // Syntax: TYPE type-character [second-type-character]
    //
    // Sets the type of file to be transferred. type-character can be any
    // of:
    //
    // * A - ASCII text
    // * E - EBCDIC text
    // * I - image (binary data)
    // * L - local format
    //
    // For A and E, the second-type-character specifies how the text should
    // be interpreted. It can be:
    //
    // * N - Non-print (not destined for printing). This is the default if
    // second-type-character is omitted.
    // * T - Telnet format control (<CR>, <FF>, etc.)
    // * C - ASA Carriage Control
    //
    // For L, the second-type-character specifies the number of bits per
    // byte on the local system, and may not be omitted.

    final Set<Character> acceptedTypeCharacters = new HashSet<Character>(Arrays.asList(
            new Character[] {'A','E','I','L'}
    ));

    final Set<Character> acceptedOptionalSecondCharacters = new HashSet<Character>(Arrays.asList(
            new Character[] {'N','T','C'}
    ));

    if( acceptedTypeCharacters.contains(typeCharacter) ) {
        if( new Character('A').equals( typeCharacter ) || new Character('E').equals( typeCharacter ) ){
            if( acceptedOptionalSecondCharacters.contains(optionalSecondCharacter) ) {
                executeCommand("TYPE " + typeCharacter + " " + optionalSecondCharacter );
            }
        } else {
            executeCommand("TYPE " + typeCharacter );
        }
    }
}